High-power diode lasers can be used as pump sources for conventional solid-state lasers, thin-disk lasers, and fiber lasers due to their high electro-optic efficiency, narrow spectral width, and high beam quality. For such applications, long lifetimes (for example, exceeding 30,000 hours), reliable and stable output, high output power, high electro-optic efficiency, and high beam quality are generally desirable. Such performance criteria continue to push diode laser designs to new performance plateaus.
Because modern crystal growth reactors can produce semiconductor materials of very high quality, the long-term reliability of high-power diode lasers can depend strongly on the stability of the laser facets. Although facet stability is generally better for conventionally-coated Al-free materials than for AlGaAs materials, high-power Al-free GaAs lasers operating at wavelengths less than one micron nevertheless suffer from facet degradation that compromises the reliability of the diode laser by causing short and long-term decreases in the performance criteria of the diode.
Laser facet degradation is a complex chemical reaction that can be driven by light, current, and heat, and can lead to short-term power degradation during burn-in, long-term power degradation during normal operation, and, in severe cases, to catastrophic optical mirror damage (COMD). Complex oxides and point defects present on a cleaved surface of a diode laser can be trapped at the interface between the reflective coating and the semiconductor material. As current is applied to the device, charge carriers can diffuse toward the facet as the surface acts as a carrier sink due to the presence of states within the band gap created by point defects and oxidation of the surface. Light emission from the diode can photo-excite the carriers at the facet surface, resulting in electron-hole pair generation, and charges generated from the electron-hole pairs can electro-chemically drive an oxidation reaction at the facet. Additionally, non-radiative recombination can occur, resulting in point defect motion and localized heating. Heating of the semiconductor material can induce thermal oxidation at the facet, further increasing the absorbing oxide layer thickness formed at the semiconductor-oxide interface.
In other situations, native oxides on GaAs and related semiconductor compounds generally stratify, leaving mostly GaO near the surface of the compound. Elemental arsenic can precipitate at the semiconductor-oxide interface either as island-like point defects or as a uniform layer. The metallic-like arsenic defects are strong absorbing centers and are believed to contribute to light absorption at the facet. As the oxidation reaction at the surface continues, the total absorption by the interface layer increases as the facet region heats up, significantly reducing the band gap energy at the facet, and leading to thermal runaway.